A digitizer is an electronic input device comprising a surface and a plurality of sensors. When a stylus is moved over the digitizer surface, the position of the stylus point is detected by the sensors underlying the surface of the digitizer, and an image typically is produced on an output device, such as a computer monitor. (Alternately, the stylus may include a sensor for detecting emissions from the digitizer.) The sensors may be, for example, optical, electromagnetic or pressure sensitive. Such devices are often used in graphics tablets that can be placed near a computer monitor to allow a person to write or draw with a pen on the tablet, and have the drawn image appear on the computer screen.
One shortcoming of such systems is that the correlation between locations on the tablet writing surface and the image on the screen is difficult to determine. Thus, if a user wishes to add a line to a drawing on a screen, he may not know where to position the stylus on the digitizer. While such systems work well for rough sketching or for capturing a person's signature, for example, more detailed writings or drawings are difficult to produce without significant trial and error.
More recently, digitizers have been combined with liquid crystal displays (“LCD's”) mounted over or under a digitizer, and aligned therewith so that a stylus creates an image on the LCD directly below where the stylus is placed. A protective layer of material is generally placed over the LCD to keep it from being damaged by the stylus. This gives the user the impression of writing directly on the LCD, and allows the user to remove the stylus from the device momentarily and know exactly where to re-place the stylus when he resumes work on a drawing. Such combined digitizers and LCD's are commonly seen in hand-held personal digital assistants (PDA's) and are also found in drawing tablets such as the Cintiq brand combined monitor and writing tablet available from Wacom Co., Ltd.
The image on the screen of a typical LCD becomes distorted when pressure is applied to the LCD—against the polarizer that forms the top layer of a typical LCD, for example. Therefore, when an LCD is used with a digitizer to make an electronic input/output device as described above, it is necessary to mount a protective layer of material above the top of the LCD. This layer is often formed from a non-rigid material that will be deflected a certain distance when pressure is applied by the stylus, such as during writing, drawing, etc. The distortion may be referred to as “puddling” due to its appearance. Therefore, it is necessary to space the protective layer from the LCD by a distance greater than this deflection distance to keep the protective layer from contacting the LCD when a stylus is used. However, it is important to minimize the gap between the polarizer and the protective layer for two reasons. First, the stylus must be relatively close to the digitizer to be detected, and, in many applications, the digitizer is already spaced from the stylus by the thickness of the LCD. Second, because the protective layer has a refractive index different from that of air, the air in the gap or space between the LCD surface and the digitizer top surface will create parallax errors when a user views the screen at an angle, as described in more detail below.
FIG. 1 is a representation of an image on a screen of a prior art LCD in an input/output device 8. FIG. 2 shows the screen input/output device 8 with a stylus pressing thereagainst. As can be appreciated from FIG. 2, the image on the screen is distorted significantly when pressure is applied, and has the puddling created by application of force to the polarizer material. FIG. 3 illustrates the protective layer of material over the LCD, so that a stylus presses against the protective layer rather than the LCD surface. The protective layer is formed of a material such as acrylic, that provides a good “feel” to a user writing with a stylus, that is, a material which resists the movement of the stylus point much like a sheet of paper resists the movement of a pen. The protective layer may be made of a rigid material, such as glass, that would have a very small deflection, but such material tends to scratch, is too fragile for general use, and does not provide a desirable “feel.” Thus protective layers formed from preferred materials tend to flex when pressure is applied to them, and the gap between the protective layer and the LCD must be large enough so that the protective layer does not contact the LCD when a user writes on it. The protective plate and a gap between the plate and LCD increases the thickness of the input/output device, and also increases its manufacturing cost.
FIG. 3 also illustrates another problem caused by the gap between the LCD and the protective layer, namely the parallax error that occurs when using input/output devices with a protective layer spaced from an LCD by a gap. The display device comprises a digitizer 10, a backlight 11 mounted on the digitizer, and an LCD panel 12 mounted on the backlight. The LCD panel comprises a plurality of layers, namely, from bottom to top, a glass base 14, liquid crystal material 16, a color filter 18, a second glass base 20 and a polarizer 22. A protective panel 24 having a top surface 26 is mounted above LCD 14 and spaced therefrom by a gap 28. The distance between the top surface of the protective layer and the top of the color filter 18 where an image is created is denoted by the letter “h.”
A stylus 30 is used to write on the input/output device. (The stylus 30 is preferably cordless and batteryless such as a stylus available from Wacom Co., Ltd., and disclosed in U.S. Pat. No. 4,848,553.) As will be appreciated from FIG. 3, when stylus 30 contacts top surface 28 at point 32, a spot is created at point 34 directly beneath point 32. A ray of light is schematically shown by line 40 between point 34 and the user. Due to the different indices of refraction of the glass in the LCD, the layer of protective material and the air in the gap and surrounding the unit, point 34 will appear to the user to be located at point 38 on the surface 28 of the protective layer at a distance d from point 32 rather than at point 32 under the tip of the stylus. This error becomes more pronounced at greater viewing angles, and decreases the accuracy with which a user can position the stylus.
It would therefore be desirable to provide an input/output device that minimizes parallax error by reducing the gap between the protective layer and the LCD without causing image distortion on the LCD when a stylus is used to write on the device.